Some exceedingly interesting experiments might be made by means of a telescope having a prism attached to its objective extremity, and furnished with a micrometer, because by it the difference of the illumination of objects might be determined with extreme accuracy—as for example, the comparative intensity between the bright and dark parts of the moon, the comparative intensity of the solar light reflected by the moon, and the lumière cendré, or the light of the earth reflected on the moon, whence a comparison might be made between the light of the sun and that of the earth. Hence also it might be known whether the terrestrial hemispheres successively visible from the moon are more or less luminous, according as they contain more land or water, and at the same time it might be possible to appreciate the more or less cloudy or clear state of our atmosphere, so that in time we might ultimately find in the lumière cendré of the moon data upon the mean diaphaneity of different terrestrial hemispheres which are of different temperatures. It is found by experience that heat is developed in opaque and translucent substances by their absorption of solar light, but that the sun’s rays do not sensibly alter the temperature of perfectly transparent bodies through which they pass. As the temperature of the pellucid planetary space can be but little affected by the passage of the sun’s light and heat, neither can it be sensibly raised by the heat now radiated from the earth. Doubtless the radiation of all the bodies in the universe maintains the ethereal medium at a higher temperature than it would otherwise have, and must eventually increase it, but by a quantity so evanescent that it is hardly possible to conceive a time when a change will become perceptible.